D. Borba 1 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Excitation of...

D. Borba 1 21st IAEA Fusion Energy Conference, Chengdu China 21st October 2006

Excitation of Alfvén eigenmodes with sub-Alfvénic neutral beam ions in JET and DIII-D plasmas

Duarte Borba on behalf of JET EFDA contributors

in collaboration with

Raffi Nazikian on behalf of the DIII-D program


Outline

• Objectives– Determine experimentally the Vb / VA threshold for the excitation of

Alfvén cascades (RSAE) using Neutral Beam Injection (NBI)

• Requirements– Core fluctuation measurements required due to high toroidal mode

number (n) of instabilities

• Setup– Low energy beams with high magnetic field to minimise Vb / VA

• Experiments in JET and DIII-D– Scan of the magnetic field, beam energy and direction and major radius

• Implications and Conclusions– Further opportunity to diagnose the safety factor profile (q-profile)– Core plasma turbulence and transport studies


Outline









Objective of the experiment

Alfvén continuum deep reversed shearAlfvén continuum deep reversed shear

TAE~A/2

RSAE<<A

• Condition for instabilities requires:– Efficient energy exchange

between the particles and the wave (resonance condition)

+ p t or b + n d = 0

for TAEs VA = Vb also (Vb = VA/3)

for Cascades (RSAE) VA >> Vb

Objective of the experiment: Determine experimentally the Vb/VA

threshold for the excitation of Alfvén cascades (RSAE)

S.E.Sharapov et al., Nuclear Fusion v.46 (2006) S868.


Outline









Diagnostic Requirements

Condition for instabilities requires:Efficient energy exchange between the particles and the waves

Wave

ParticleRadial Mode width m > Radial Orbit width p

m∞ 1/ (n s)

Toroidal mode number Magnetic Shear m

pIn reversed shear scenarios, around the zero shear (s=0) point, large toroidal mode number

(n) waves can be excited

Core localised Waves with large n are invisible for external diagnostics (magnetics), so internal measurements are required


Diagnostic Setup (JET)

• Far infrared DCN interferometer 3 vertical chords, measured line-integrated neL fluctuations out of the 4 available channels

• Microwave reflectometer radially localized measurement of density fluctuations using O-mode and X-mode

• Magnetic probes New diagnostic capability allows core

fluctuation measurements, essential for the detection of NBI driven cascades


Diagnostic Setup (DIII-D)

CO2 interferometer, ~ 1.6 MHz bandwidth, 4 chords, line-integrated neL

65 and 42 GHz Quad-reflectometer, ~10 MHz bandwidth, radially localized measurement of density fluctuations

Beam-Emission Spectroscopy (BES), 500 kHz bandwidth, providing spatially localized measurements of ne with increased sensitivity

Far-Infrared Scattering (FIR), ~10 MHz bandwidth, line-integrated measurement of low-k density fluctuations (k = 0 - 2 cm-1 )

CO2 Interf.

65 GHz42 GHz


Measurements at JET

• Far infrared DCN interferometer most versatile data gives information on the instabilities throughout the shot• Microwave reflectometer (O-mode) working as an interferometer gives the best signal, but relies on the density to be such that the diagnostic frequency is above the cut off Magnetic probes are mostly sensitive to low toroidal mode number instabilities (n=2,3,4), due the radial extent of these modes

Using these diagnostics allows an detailed analysis of the instability threshold in different configurations

Microwave reflectometer

Magnetic probe

Far infrared DCN interferometer

3.0 Time (s) 4.4

Fre

que

ncy

(kH

z) 20

20

20

120

120

120

# 67674

S. Sharapov et al Phys. Rev. Lett. 93, 165001 (2004); Berk et al, Phys. Rev. Lett. 87, 185002 (2001)


Measurements at JET

time (s)

Alfvén Cascades near r/a=-0.2 using X-mode Reflectometer# 67732


Outline









Experimental Scenario JET

• Maximum Magnetic field Bt=3.45 Tesla

• Lowest beam energy possible

• Lower Hybrid Current drive to obtain reversed shear (PLHCD=2MW)

• Low density to maximize the fraction of

fast ions

Objective: minimise Vb/VA ~ Ebeam 0.5 ne

0.5 /BT in order to find the

threshold for the excitation of Alfvén cascades (RSAE)


Difficult to reduce velocity ratio Vb/VA below 0.4 in DIII-D

Experimental Scenario DIII-D

Objective: minimise Vb/VA ~ Ebeam0.5

ne0.5

/BT in order to find the threshold for the excitation of Alfvén cascades (RSAE)

0 0.2 0.4 0.6 0.8 1Time (s)

Vb/VA

PNBI (MW)

03

0.00.40.8

ne5 1019

3 1019

# 125961 #125975


Outline









Neutral Beam energy scan (JET)

117 keV Å 0.27 VA

No beams

80 keV Å 0.22 VA

50 keV Å 0.17 VA

Fre

quen

cy (

kHz)

Fre

quen

cy (

kHz)

Time (s) Time (s)3.0 3.2 3.4 3.6 3.8 3.0 3.2 3.4 3.6 3.8

3.0 3.2 3.4 3.6 3.8 3.0 3.2 3.4 3.6 3.820

40

60

80

100

120

140

20

40

60

80

100

120

140

20

40

60

80

100

120

140

20

40

60

80

100

120

140

Cascades clearly visible with 50 keV beams at JET

• Beam energy scan performed at JET using low (80 keV) and high (140 keV) voltage NBI

• The energy of the 80 keV NBI was also decreased to 50 keV

Microwave reflectometer (working as a interferometer)# 66957 # 66962

# 66963# 66964


50 keVVb/VA≈0.45

80 keVVb/VA≈0.60

Cascade Modes Observed with 50 keV Beam injection in DIII-D though at reduced level and number compared to 80 keV

Neutral Beam energy scan (DIII-D)

# 125961

# 125975

R. Nazikian et al Phys. Rev. Lett. 96, (2006), M. Van Zeeland et al Phys. Rev. Lett. 97, (2006).


Fre

qu

en

cy (

kH

z)F

req

ue

ncy (

kH

z)

10

60

110

160

210

260

10

60

110

160

210

260

Time (s)0.2 0.4 0.6 0.8 1.0 1.2

0.2 0.4 0.6 0.8 1.0 1.2

Pco/Pco+Pctr=1.0

Pco/Pco+Pctr=0.20

Alfvén Eigenmode Excitation is Sensitive to Beam Ion Direction in DIII-D

• Reverse Shear Alfvén modes excited mostly by co-injected ions in DIII-D

• Difference in drive is either due to finite orbit width effects, the radial distribution of co/counter ions and/or on an intrinsic sensitivity of the mode to ion direction.

• Results suggest that modes can drive current in fusion plasma from the preferential redistribution of co going alpha particles

Neutral Beam direction (DIII-D)

#126597

#126581

B.N.Breizman et al., Physics of Plasmas v.10 (2003) 3649


Outline









q-profile

4 5 Time (s) 6 7

Fre

que

ncy

(kH

z)

100

200

50

150 qmin = integer

qmin = integer


t=4.8 s

Fre

quen

cy (

kHz)

20

120

t=7.2 s

Fre

quen

cy (

kHz)

20

q-profile evolution (JET)

q=4 Cascade

q=3 Cascade

q=2 Cascade

t=3.44 s

Fre

quen

cy (

kHz)

20

120

120

- Information from the Alfvén waves is used to reconstruct the q-profile evolution more accurately and validate the Motional Stark Effect (MSE) measurements


50 keV @ Vb/VA≈0.45

qmin=4 qmin=3 qmin=2

Cascade Modes with 50 keV Injection also clearly indicate rational q-min crossings in DIII-D

q-profile evolution (DIII-D)

#125975


Conclusions I

• Alfvén waves (RSAE/ Alfvén Cascades), can be excited by sub-Alfvén Neutral Beam Injection as low as Vb/VA~0.17 (JET).

• Observations suggest that the interaction of these modes with the energetic particles may impact the current drive in vicinity of qmin in advanced tokamak regimes using NBI

• Alfvén eigenmode excitation is sensitive to NBI

direction (DIII-D)

• Results suggest that modes may drive current in vicinity of qmin in burning plasmas from the preferential redistribution of co-passing alpha particles


Conclusions II

• Alfvén cascades (RSAE) will be destabilised in ITER reversed shear auxiliary heated discharges

• Possible redistribution of the Helium ash from the plasma core• Possible current drive in the vicinity of qmin surface• Powerful tool to diagnose the evolution of the qmin surface

• Diagnostic setup in ITER must be able to measure

Alfvén instabilities in the plasma core

D. Borba 1 21 st IAEA Fusion Energy Conference, Chengdu China 21 st October 2006 Excitation of...

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